1. Field of the Invention
The present invention relates, in general, to a ferroelectric capacitor and, more particularly, to an improvement in fatigue and leakage current along with the ferroelectric capacitor. Also, the present invention is concerned with a method for fabricating the same.
2. Description of the Prior Art
For ferroelectric capacitor, an electrode with superior conductivity is necessary to transfer charges. Typically, metal electrodes or conductive oxide electrodes have been employed in ferroelectric capacitors. As for metal electrodes, illustrative are a platinum electrode and a platinum/titanium electrode. They are enough only for the purpose of conductivity. However, they are disadvantageous in that the electrical properties of the ferroelectric capacitors comprising only the metal electrodes become degraded as follows. Oxygen vacancies are accumulated at an interface between the ferroelectric and the metal electrode, leading to formation of space charge region at the interface. Since such space charge region has a characteristic of linear capacitor, the dielectric properties of the capacitor are determined by the space charge rather than by the ferroelectric. This causes a rapid fatigue which results from the polarization reversion of ferroelectric, degrading the electrical properties.
In order to prevent the formation of space charge region attributable to the accumulation of oxygen vacancy, there have been suggested to replace Pt electrode with conductive oxide electrode. A RuO.sub.2 electrode, representative of the conductive oxide electrodes, was reported to be able to improve the fatigue somewhat because of supplying oxygen to interface region. However, there appears a significant increase of leakage current compared to that of the metal electrode. This is believed to be attributed to a fact that defect states present at the interface between the ferroelectric and the conductive oxide metal lower the barrier height of electron or that, upon forming PZT, Ru metal from RuO.sub.2 would diffuse and react with PbO at the grain boundary of PZT to form a conductive lead ruthenate (PbRuO.sub.3-x), which results in a reduction of electrical resistance in the ferroelectric thin film. Besides, the conductive oxide electrodes are of lower polarization than metal electrodes as well as of high coercive field. Thus, capacitor adopting only conductive oxide electrodes seems to be difficult to apply for memory devices.